Bragg grating device for measuring an acceleration

ABSTRACT

The Bragg grating device for measuring an acceleration, has at least two optical Bragg gratings ( 11, 12 ), each formed in elastic material, for supplying optical radiation (S) and at least one deflectable mass (M) connected to both gratings for generating an inertial force that is dependent on the acceleration which acts upon the device, in order to produce elastic extension of one of the two gratings and simultaneous elastic contraction of the other grating. The device is also suitable for vibration frequency measurement.

FIELD OF THE INVENTION

The invention relates to a Bragg grating device for measuring anacceleration.

BACKGROUND OF THE INVENTION

The paper by T. A. Berkoff, A. D. Kersey: “Experimental Demonstration ofa Fiber Bragg Grating Accelerometer”, IEEE Photonics Technology Letters,Vol. 8, No. 12, December 1996, pages 1677-1679, discloses a Bragggrating device for measuring an acceleration which has:

an optical Bragg grating, formed in elastic material, for supplyinglight, and

a deflectable mass connected to the grating for generating an inertialforce that is dependent on the acceleration which acts on the mass, inorder to produce elastic extension of the grating.

The connection between the mass and the grating is produced by aresilient layer, which is supported by a fixed baseplate and in whichthe grating is embedded. The extension of the grating is produced by themass moving, in particular vibrating, at right angles to the directionof this extension.

The known device is operated as follows: light from a broadband sourceis supplied to the grating. The grating reflects a proportion of thelight supplied at a grating-specific Bragg wavelength, which changeswith the extension of the grating. The extension of the grating isproduced by the inertial force generated by the mass, which isproportional to the acceleration that acts on the device and is to bemeasured. The reflected proportion of the light is supplied to anevaluation device, which determines which Bragg wavelength is containedin this proportion.

The evaluation device has a Mach-Zehnder interferometer having two armsof mutually different optical length. The reflected proportion of thelight is coupled into the two arms, superimposed after passing throughthe two arms and is brought to interference and then fed to a detector.The determination of the Bragg wavelength contained in the reflectedproportion is carried out with the aid of a phase modulator arranged inone of the two arms for the phase modulation of the part of thereflected proportion of the light that is guided in this arm relative tothe part of this proportion which is guided in the other arm.

In the publication by J. R. Dunphy: “Feasibility study concerningoptical fiber sensor vibration monitoring subsystem” in SPIE Vol. 2721,pp. 483-492, a Bragg grating device for vibration monitoring isdescribed, which has at least one optical Bragg grating formed inelastic material for supplying light. In this case, evaluation devicesfor measuring the grating-specific Bragg wavelength of the grating areconsidered and compared with each other, said devices having aspectrometer, an interference filter, a tunable fiber grating, ascanning Fabry-Perot filter, a wavelength-dispersive element with asampled detector or a tuned acousto-optical filter.

For a Bragg grating device having four Bragg gratings, among theseevaluation devices, one device is viewed as relatively advantageouswhich, for each grating, has a tuned acousto-optical filter forevaluating the proportion of the light supplied and reflected by thisgrating with regard to the grating-specific Bragg wavelength containedin this proportion.

The publication by L. Zhang et al: “Spatial and Wavelength MultiplexingArchitectures for Extreme Strain Monitoring System usingIdentical-Chirped-Grating-Interrogation Technique”, in 12thInternational Conference on Optical Fiber Sensors, Oct. 28-31, 1997, pp.452-455, reveals that the reflectivity R of a Bragg grating is afunction both of the wavelength and also of the stress on the grating.

In this case, in a limited wavelength range around a grating-specificcentral Bragg wavelength, the reflectivity is greater than zero andessentially equal to zero outside this range. As the stress changes, theentire wavelength range is displaced linearly.

When “chirped” Bragg gratings are used, that is to say gratings with avarying grating constant, a reflectivity R of the grating can beobtained which depends on the wavelength approximately in the form of arectangular curve, so that the reflectivity within the wavelength rangeis substantially constant.

SUMMARY OF THE INVENTION

The object of the invention is to provide a Bragg grating device formeasuring on acceleration which permits particularly simple evaluation.

To solve this problem, the Bragg grating device for measuring anacceleration according to the invention has at least two optical Bragggratings, each formed of elastic material, for supplying opticalradiation, and at least one deflectable mass connected to both gratingsfor generating an inertial force that is dependent on the accelerationwhich acts upon the device, so as to produce elastic extension of one ofthe two gratings and simultaneous elastic contraction of the othergrating.

The solution achieved by the present invention applies, inter alia, tothe case that often occurs in which on one occasion, the inertial forcethat is exerted produces a state of deformation of the two gratings inwhich one of the two gratings is extended elastically while, at the sametime, the other grating is contracted elastically and, on anotheroccasion, a state of deformation of the two gratings which is differentfrom this state of deformation is produced, in which, conversely, theother grating is extended elastically and, at the same time, the onegrating is contracted elastically. In particular in the case ofaccelerations in the form of vibrations, the case therefore occurs inwhich the one and the other states of deformation of the two gratingsalternate.

As used herein, elastic material means a material in which the inertialforce generated can produce such a large elastic deformation that achange produced by this deformation in a grating constant of the Bragggrating produces a measurable change in the grating-specific Braggwavelength of a grating.

Deflectable mass as used herein means a mass which can be moved relativeto the two gratings that are accelerated by the acceleration to bemeasured, for example relative to an accelerated frame to which the twogratings are fixed.

The inertial force produced by the mass can act on the two gratingsdirectly or indirectly, for example via a force transmission device.

The two Bragg gratings are preferably arranged one behind another in apropagation direction of the optical radiation supplied. For example,these two gratings can be formed one behind another in the propagationdirection in an optical conductor made of elastic material for guidingthe radiation. The conductor can be, in particular, an optical waveguide, for example an optical glass fiber used in conventional Bragggrating sensors.

In a preferred and advantageous refinement of the device according tothe invention, the mass is arranged between the two gratings andconnected directly to each of the two gratings so that the inertialforce produced by the mass acts directly on the gratings.

The mass can also be connected indirectly to the gratings via a forcetransmission device in order to convert the inertial force produced bythe mass into a force that acts on each grating in such a way that oneof the two gratings is extended and, at the same time, the other gratingis contracted. In this case, the inertial force acting indirectly on thegratings and/or the extension or contraction of the gratings can beenlarged or reduced.

It is expedient if the force transmission device generates a force whichis oriented in the propagation direction and/or the opposite directionand which acts directly on one grating in an extending manner and on theother grating in a contracting manner. For example, in this case theforce transmission device can have a lever which can rotate about anaxis of rotation that is substantially fixed relative to the device, oranother mechanism that acts appropriately.

The two gratings can be formed or prepared in such a way that, in thestate in which they are free of inertial force, that is to say in theacceleration-free state of the device, they have the same central Braggwavelengths. With regard to the possibility of utilizing an advantageouslinear characteristic curve range brought about by the two gratings, itis advantageous, however, if the two gratings have mutually differentcentral Bragg wavelengths in the inertial-force-free state, the centralBragg wavelength of one of the two gratings lying within a gratingbandwidth of the other grating.

In a refinement of the device according to the invention, at least oneof the two Bragg gratings has a fixed grating constant. A particularrefinement is in this case distinguished by a fixed grating constant ofeach of the two gratings. An improved linear characteristic curve rangecan be obtained with a refinement in which at least one of the two Bragggratings has a variable grating constant (chirped grating), it beingadvantageous and expedient if both gratings each have a variable gratingconstant.

One advantage of the device according to the invention is to be seen inthe fact that, during the evaluation of the proportion of the suppliedoptical radiation, coming from the two gratings, with respect to theirgrating-specific Bragg wavelengths, it is possible to use a simplebroadband optical detector instead of a complicated narrow-banddetector. Accordingly, an advantageous refinement of this device has abroadband optical detector for receiving a proportion, coming from bothgratings, of the supplied optical radiation.

An advantageous method of operating a device according to the inventionhas in general terms the following steps:

supplying an optical radiation of a wavelength range containing thecentral Bragg wavelength of each of the two Bragg gratings to thegratings, and

measuring the intensity of a proportion of the supplied radiation comingfrom the gratings, as a measure of the acceleration to be measured.

An advantageous application of a device according to the inventionconsists in measuring a mechanical vibration frequency, the vibrationfrequency being lower than a resonant frequency of the device.

Another advantageous application of a device according to the inventionconsists in measuring a vibration amplitude of a mechanical vibrationfrequency which is higher than a resonant frequency of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail by way of example in thefollowing description, in conjunction with the appended drawings, inwhich:

FIG. 1 is a schematic drawing of the basic structure of a deviceaccording to the invention having two Bragg gratings,

FIG. 2 shows a characteristic curve graph of the device according toFIG. 1, the two Bragg gratings having a fixed grating constant,

FIG. 3 shows a characteristic curve graph of the device according toFIG. 1, the two Bragg gratings having a variable grating constant,

FIG. 4 shows an embodiment of the device according to FIG. 1, in whichthe mass is connected to the two gratings via a force transmissiondevice, and

FIG. 5 shows an embodiment of the invention in which a plurality ofdevices according to FIG. 1 are arranged one behind another.

DETAILED DESCRIPTION OF THE INVENTION

The Bragg grating device for measuring an acceleration, illustrated inFIG. 1 and designated generally by the reference numeral 1, has twooptical Bragg gratings 11 and 12 each formed of elastic material forsupplying optical radiation S, and a deflectable mass M connected toboth gratings 11 and 12 for generating an inertial force that isdependent on the acceleration which acts on both gratings 11 and 12. Themass M is connected to the gratings so as to produce elastic extensionof one of the two gratings 11 or 12 and simultaneous elastic contractionof the other grating 12 or 11.

The optical radiation S generated by a radiation source 13 is propagatedin a direction designated by x in FIG. 1, for example, parallel to thedirection at which the acceleration to be measured and designated by aalso acts on the two gratings 11 and 12. For example, the acceleration amay be a component of an acceleration which acts on the two gratings 11and 12 at an angle different from zero in relation to the direction x.

The two Bragg gratings 11 and 12 are preferably arranged one behindanother in the propagation direction x, so that a proportion (notillustrated) of the radiation S incident on grating 11 and which haspassed through the grating 11, is incident on the grating 12.

For example, the two gratings 11 and 12 are arranged at a distance dfrom each other in the direction x. In this case, the mass M can bearranged, for example and as indicated in FIG. 1, between the two Bragggratings 11 and 12 and can be connected directly to each of the twogratings 11 and 12.

If the two gratings 11 and 12 are, for example, accelerated in thedirection x with the acceleration a to be measured, the mass M deflectedparallel to this direction x relative to the two gratings 11 and 12,generates an inertial force −F=M·a, directed opposite to the directionx, and which acts in a contracting manner on the grating 11 and, at thesame time, in an extending manner on the grating 12.

If, on the other hand, the two gratings 11 and 12 are accelerated in thedirection opposite to the direction x with the acceleration −a to bemeasured, then the mass M generates an inertial force F=−M·a, which actsin the direction x and acts in an extending manner on the grating 11and, at the same time, in a contracting manner on the grating 12.

Since each of the two gratings 11 and 12 is formed of elastic material,the system jointly comprising the two gratings 11 and 12 and the mass Mconnected to them can be considered as a mechanical system whichconsists of two springs 11 and 12 that are elastic in the direction xand the mass M that is connected to these springs, each of the twosprings 11 and 12 having a spring constant k1 and k2, respectively,determined by the respective elastic material.

If the two gratings 11 and 12 are formed in material with the sameelasticity, then k1=k2. This is the case, for example, if, as indicatedin FIG. 1, the two gratings 11 and 12 are formed on a common opticalconductor 10 extending in the direction x, and which consistssubstantially homogeneously of a single elastic material, and in whichoptical radiation S is supplied to the grating 11 and a proportion ofthis radiation S that has passed through this grating 11 is supplied tothe grating 12.

For example, the conductor 10 is an optical fiber made of glass orplastic, in which the gratings 11 and 12 are formed in a known way. Themass M can be connected to the conductor 10 between the gratings 11 and12.

Each of the two gratings 11 and 12 has a plurality of grating lines 100following one another in the direction x and a grating constant cdetermined by the distance between two respectively adjacent gratinglines 100 in the respective grating, the grating constant increasingwhen the grating 11 or 12 is extended in or counter to the direction x,and decreasing when the grating 11 or 12 is contracted in or counter tothe direction x. Accordingly, in each grating 11 and 12, the centralBragg wavelength λ0 or λ0′ of the respective grating, which depends onthe grating constant c and is therefore grating-specific, is displacedin a specific direction, as is known, when the grating 11 or 12 isextended, and is displaced in the direction opposite to this specificdirection when the grating 11 or 12 is contracted.

Since, in the present case, one of the two gratings is extended and, atthe same time, the other grating is contracted, the respectivegrating-specific central Bragg wavelengths λ0 and λ0′ of the twogratings 11 and 12 are displaced in mutually opposite directions.

A grating 11 or 12 can have a fixed or variable grating constant c. Inthe case of a fixed grating constant c, this constant c, that is to saythe distance between two respective grating lines 100 following eachother in the direction x, is constant, while in the case of a variablegrating constant c, this distance changes.

If two gratings 11 and 12 with the same reflectivity R and a fixedgrating constant c in each case are used, the characteristic curve graphshown in FIG. 2 is significant.

In this graph, the difference λ−λ0 between an optical wavelength λ andthe central Bragg wavelength λ0 of the grating 11 is plotted on theabscissa, and an optical intensity I is plotted on the ordinate.

The curve A in this graph indicates the reflectivity R of the grating11, which depends on the wavelength difference λ−λ0, assuming thatgrating 11 is considered on its own, without taking grating 12 intoaccount, and is irradiated with optical radiation S at the wavelength λ.The reflectivity R gives the intensity I of the proportion S1 of theoptical radiation S supplied to this grating 11, reflected by grating 11considered alone, as a function of the wavelength different λ−λ0. In thecase of a fixed grating constant c of the grating 11, the reflectivity Rcan be assumed to follow a Gaussian curve or a similar bell-shaped curvewhich has a maximum value R0 at λ−λ0=0. The same is true of the grating12 of the Bragg wavelength λ0′, irradiated with radiation S ofwavelength λ, in which the reflectivity R, that is to say the intensityI that depends on the wavelength difference λ−λ0′ of the proportion S1of the supplied optical radiation S, reflected by this grating 12considered alone, in the case of a fixed grating constant c of thisgrating 12, can likewise be assumed to be a Gaussian curve or similarbell-shaped curve which has a maximum value at λ−λ0′=0. In FIG. 2, thecurve of the reflectivity R of this grating 12 is not shown, forsimplicity.

The bell-shaped curve of the reflectivity R of each such Bragg grating11 and 12 defines a specific grating bandwidth of this grating in eachcase, which can be assumed to be the half-value width of this curve, forexample. The half-value width is the distance between the two points onthis curve which lie at half the maximum value of the curve of thereflectivity R which, in the case of the grating 11, is the curve A inFIG. 2, at which half the maximum value is equal to R0/2 and the gratingbandwidth is designated by Δ.

For the previously described curve of the reflectivity R of each grating11 and 12 it is additionally assumed that the respective grating 11 or12 is in the state free of inertial force, so that in this state thepoint λ−λ0=0 or λ−λ0′=0 on the abscissa is located specifically at thecentral Bragg wavelength λ=λ0 or λ=λ0′ of the grating 11 or 12.

If the grating 11 or 12 is moved out of the state free of inertial forceas a result of extension and/or contraction parallel to the direction x,the entire curve of the reflectivity R, that is to say the curve A inthe case of grating 11, is displaced to the right or left along theabscissa, so that following this extension and/or contraction, themaximum value of the curve of the reflectivity R of the grating 11 or 12is located at λ−λ0±Δλ0=0 or, respectively, λ−λ0′±Δλ0′=0, the sign of Δλ0and Δλ0′ being determined in accordance with whether there is extensionor contraction of the grating 11 or 12, and the magnitude of Δλ0 andΔλ0′ depending on the extent of this extension or contraction.

In the case of the curve B of FIG. 2, it is assumed that both gratings11 and 12 are present, that optical radiation S is supplied to thegrating 11 and that the proportion of the radiation S let through by thefirst grating 11 is incident on the second grating 12. Furthermore, alight source 13 is assumed which generates at least that wavelength λ ofthe grating bandwidth of each grating 11 and 12, both in the state freeof inertial force and in each extension and/or contraction state whichmay occur in this grating 11 or 12.

Although a source 13 generating these wavelengths one after another overtime is not ruled out in principle, it is advantageous to use abroadband source 13, which is much simpler to implement, since all thesewavelengths are then available simultaneously.

Assuming a broadband source 13, curve B indicates the intensity I,integrated over all the wavelengths λ of the broadband source 13, of thereflected proportion S1 of the broadband optical radiation S supplied tothe grating 11, originating from both gratings 11 and 12 and coming backfrom the grating 11 in the direction opposite to the direction x,depending on the difference λ0′−λ0 between the selected central Braggwavelength λ0′ of grating 12 and the selected central Bragg wavelengthλ0 of grating 11.

In the present example of the respectively fixed grating constants c ofthe gratings 11 and 12, curve B is an inverse bell-shaped curve and hasa minimum value R1 at λ0′−λ0=0, said minimum value depending on themaximum value R0 of the curve of the reflectivity R of the grating 11.

On the left and right of this minimum value R1 the curve B has a curvebranch B1, B2 in each case which rises approximately linearly for somedistance from this value R1.

If the two gratings 11 and 12 are selected, for example, such that whenfree of inertial force they each have the same fixed grating constant c,the same central Bragg wavelength λ0=λ0′ and the same maximum value R0at the Bragg wavelength λ0=λ0′, the operating point of the two gratings11 and 12 is located at the minimum value R1 of the curve B, that is tosay simultaneous extension of one and contraction of the other of thetwo gratings 11 and 12 produces a change in the integrated intensity Iof the reflected proportion S1 corresponding to the curve B only in thevicinity of this minimum R1. This operating point is suitable forvibration measurements, in which only the amplitude of the oscillationis of interest.

If, by way of contrast, gratings 11 and 12 are used which, in the statefree of inertial force, have mutually different central Braggwavelengths λ0≠λ0′, the common operating point of the two gratings 11and 12 can be moved into the approximately linear oblique part of thecurve branch B1 or B2 of the curve B, for example to the point P of thecurve branch B2 which is illustrated in FIG. 2 and located at λ0′−λ0>0,at which the reflected proportion S1 has the integrated intensity I_(p).

If, at an operating point selected in this way, the two gratings 11 and12 are extended and contracted at the same time in opposed directions,the difference λ0′−λ0 between the central Bragg wavelength λ0′ of thegrating 12 and the central Bragg wavelength λ0 of the grating 11 changesas a function thereof, as a result of which the operating point on thecurve B is displaced in one or the other direction with respect to theoperating point in the inertial-force-free state of the gratings 11 and12. In a corresponding way, the integrated intensity of the reflectedproportion S1 changes, the change in the integrated intensity beingproportional to the slope of the curve B at the inertial-force-freeoperating point.

With reference to the exemplary inertial-force-free operating point P inFIG. 2, a change in the difference λ0′−λ0 by ±Δλ produces a change ofI_(p) by ±ΔI=b·(±Δλ), b signifying the positive slope of the curvebranch B2 at the point P.

It is ideal to place the operating point in or at least in the vicinityof a point of inflection present in the respective curve branch B1 orB2, since this curve branch B1 or B2 is particularly linear in thevicinity of this point.

In practice, sufficient linearity is achieved, for example, if themagnitude |λ0′−λ0| is about ⅓ of the grating bandwidth Δ of the grating11. The measurable extensions and contractions of the gratings 11 and 12lie in the range of Δ/4, so that, for example, with a grating 11 withΔ=350 nm at λ0=1550 nm, a maximum extension or contraction of about 100με results.

A curve B with a linearity that is improved as compared with thebell-shaped curve B of FIG. 2 can be obtained if a grating 11 and/or 12with a variable grating constant c is used. FIG. 3 shows the ideal caseof a characteristic curve graph which is achieved, at leastapproximately, with such gratings.

According to FIG. 3, the curve A of the grating 11 corresponding to thecurve A of FIG. 2 is a rectangular function, which is given by I=R0 forλ>λ0−Δ/2 and λ<λ0+Δ/2 and I=0 for all other λ, Δ being the gratingbandwidth of the grating 11.

The curve B of FIG. 3 corresponding to the curve B of FIG. 2 likewisehas a minimum value R1 at λ0′−λ0=0, said value depending on the maximumvalue R0 of the grating 11. On the left and right of this minimum R1however, this curve B has a curve branch B1 which is strictly linearbetween 0 and −Δ/2, and a curve branch B2 which is strictly linearbetween 0 and +Δ/2.

The common operating point of the two gratings 11 and 12 is moved to thelinear curve branch B1 or B2, for example to the point P on the curvebranch B2. The measurable extensions and contractions of the gratings 11and 12 also lie in the range Δ/4 here, but advantageously with a linearcharacteristic curve and a widened measuring range, since in the case ofgratings with a variable grating constant c, the grating bandwidth canbe significantly larger as compared with gratings with a fixed gratingconstant c.

In any case, the mutually different central Bragg wavelengths λ0 and λ0′of the two gratings 11 and 12 should be chosen such that the centralBragg wavelength of one of the two gratings, for example the centralBragg wavelength λ0′ of the grating 12, lies in a grating bandwidth ofthe other grating, in the example the grating bandwidth Δ of the grating11. The grating bandwidth of a Bragg grating 11 or 12 is generally arange in which the curve of the reflectivity R of this grating 11 or 12is viewed as being different from zero.

As a result of the curve B having the non-constant but approximatelylinear curve branch B1 and/or B2 over at least a section, in order toevaluate the proportion S1 of the supplied broadband optical radiation Sthat comes from the gratings 11 and 12, a broadband optical detector 14is required. The reflected proportion S1 is supplied to detector 14, forexample via an optical coupler 15, from conductor 10.

The device shown in FIG. 4 differs from the device 1 according to FIG. 1only in the fact that the mass M is not connected directly to thegratings 11 and 12 but indirectly, via a force transmission device 2 inthe form of a lever 20 that can be rotated about an axis of rotation 21that is substantially fixed relative to the device 1.

The axis of rotation 21 is at right angles to the plane of the drawingof FIG. 4 and is produced, for example, by rotatably attaching the lever20 to a frame 110 of the device 1. The mass M is fixed to a first arm201 of the lever 20, and a second lever arm 202 is connected to each ofthe two gratings 11 and 12. The ratio L1/L2 between the length L1 of thefirst lever arm 201 and the length L2 of the second lever arm 202determines the transfer function of the force transmission device 2.

An inertial force F or −F produced by the mass M and oriented in orcounter to the direction x is converted by the lever 20 into a force −Kacting in the direction opposite to the direction x or a force K actingin the direction x. Through the force −K, the grating 11 is contractedand, at the same time, the grating 12 is extended; through the force K,the grating 12 is contracted and, at the same time, the grating 11 isextended.

In the case of the device 1 according to FIG. 4, only those parts whichare important for the force transmission device 2 are shown anddescribed; all the other parts of this device 1 are the same as theparts already described and shown in the case of the device 1 of FIG. 1.

It is expedient to prestress the two gratings 11 and 12 in such a waythat each grating 11 and 12, in the state free of inertial force, isalready elastically preextended up to a certain extent. An alreadypreextended grating may be contracted elastically more easily than anon-preextended grating. Moreover, it is expedient to prestress theoptical conductor 10 elastically, so that during the contraction of agrating 11 or 12, only the elastic preextension of the conductor 10 isreduced, and no bending of the conductor 10 occurs. For example, theconductor 10 can be kept prestressed on a frame, like the frame 110 inFIG. 4.

The device 1 according to FIG. 1 or FIG. 4 will in general be operatedin such a way that the gratings 11 and 12 are supplied with opticalradiation S of a wavelength range containing the central Braggwavelengths λ0 and λ0′ of each of the two gratings 11 and 12, and theintensity I of the proportion S1 of the supplied radiation S coming fromthe gratings will be used as a measure of the acceleration to bemeasured.

Since, for example in the vicinity of the operating point of the twogratings 11 and 12, this intensity I depends linearly on the wavelengthdifference λ0′−λ0, this measured intensity I can be used directly todraw conclusions about the acceleration exerted on the two gratings 11and 12.

The device 1 according to FIG. 1 or FIG. 4 can advantageously be used asa vibration sensor for measuring a vibration amplitude, if the vibrationfrequency ω is selected to be higher than an always present resonantfrequency ω0 of the entire device 1. At these vibration frequencies ω,the deflection of the mass M is independent of the vibration frequency ωbut proportional to the vibration amplitude, which is equal to thedeflection of the two gratings 11 and 12 excited into vibration at thevibration frequency ω

The device 1 according to FIG. 1 or FIG. 4 can also be used as anacceleration meter for measuring an acceleration in such a way that thedevice 1 is excited into vibration at a vibration frequency ω, and thevibration frequency ω is less than a resonant frequency ω₀ of the device1. At these vibration frequencies ω, the deflection of the mass Mdepends on the vibration frequency ω and therefore on the acceleration.The available frequency range, both in the case of the accelerationmeter and in the case of the vibration meter, can be enlarged by meansof damping in the vicinity of resonance.

In the case of the device 1 according to FIG. 1 or FIG. 4, the two Bragggratings 11 and 12 are arranged exactly one behind another in thedirection x. However, the principles of the invention applies to devicesin which the two Bragg gratings are arranged one behind another in thedirection x and/or are arranged beside each other.

In the case of the embodiment shown in FIG. 5, a plurality of devices 1are arranged in such a way that the respective pairs of gratings, eachof which comprises the two gratings 11 and 12 connected to a mass M, aredisposed successively on the optical conductor 10.

The pairs of gratings are set to mutually different central Braggwavelengths λ0=λ1, λ0=λ2, λ0=λ3 and so on. Each pair of gratings isassigned a broadband optical detector 14 each, which is tuned to therespective Bragg wavelength λ0=λ1, λ0=λ2, λ0=λ3, respectively.

The radiation source 13 is constructed to generate broadband opticalradiation S which contains all the Bragg wavelengths λ0=λ1, λ0=λ2,λ0=λ3.

The proportion S1 of the radiation S coming from each pair of gratingsis coupled out of the conductor 10 by an optical coupler 15 and suppliedto the associated detector 14.

If an optical wavelength demultiplexer 16 is used to separate from oneanother the λ0=λ1, λ0=λ2, λ0=λ3 of the proportions S1 comingsubstantially jointly from the pairs of gratings, a single coupler 15 issufficient for the output coupling. The wavelength demultiplexer 16 canbe, for example, the grating spectrograph described in U.S. Pat. No.5,680,489, or an equivalent demultiplexer.

In the case of each device 1 described above, the proportion S1 of theoptical radiation S coming from the two gratings 11 and 12 is theproportion reflected by the gratings 11 and 12 proportion S1 can also bethe proportion passed through by the two gratings, with the evaluationarrangement modified accordingly.

What is claimed is:
 1. An acceleration measuring device comprising apair of optical Bragg gratings arranged at a distance from each otherand aligned one behind the other along an optical conductor and formedof elastic material for supplying optical radiation, a deflectable massconnected to a part of said optical conductor at said distance betweenboth gratings of said pair such that deflection of the mass in onedirection produces simultaneous elastic extension of one of saidgratings and elastic contraction of the other of said gratings anddeflection of the mass in the opposite direction produces simultaneouselastic contraction of said one of said gratings and elastic extensionof said other of said gratings, further wherein the two gratings havemutually different central Bragg wavelengths, and the central Braggwavelength of one of the two gratings lies in the grating bandwidth ofthe other grating, means for supplying the gratings with opticalradiation having a wavelength range including the central Braggwavelength of each of said pair gratings, and means for detecting theintensity of a proportion of the supplied radiation coming from thegratings as a measure of acceleration, wherein a portion of the suppliedradiation passes through the first grating of said pair of gratings, aportion of said portion of the supplied radiation passing through thefirst grating is reflected by the second grating of said pair ofgratings and reversely passes through the first part of said pair ofgratings.
 2. The device according to claim 1, wherein at least one ofthe two Bragg gratings has a fixed grating constant.
 3. The deviceaccordingly to claim 1, wherein at least one of the two Bragg gratingshas a variable grating constant.
 4. The device according claim 1,further comprising a source of optical radiation coupled to said opticalconductor and an optical detector coupled to said optical conductor forreceiving a proportion of optical radiation from said gratings.
 5. Thedevice of claim 4, wherein said source is a source of broadband opticalradiation and said detector is a broadband detector.
 6. The deviceaccording to claim 1, further comprising a plurality of said pairs ofgratings formed along said conductor and wherein a separate mass isconnected to the gratings of each said pair.
 7. A method of measuringacceleration with a device comprising a pair of optical Bragg gratings,and a deflectable mass connected to both gratings of said such thatdeflection of the mass in one direction produces simultaneous elasticextension of one of said gratings and elastic contraction of the otherof said gratings and deflection of the mass in the opposite directionproduces simultaneous elastic contraction of said one of said gratingsand elastic extension of said other of said gratings, comprising thesteps of accelerating the device into movement to deflect said mass,supplying the gratings with optical radiation having a wavelength rangeincluding the central Bragg wavelength of each said pair of gratings,and detecting the intensity of a proportion of the supplied radiationcoming from the gratings as a measure of acceleration; and furthercomprising exciting the device into vibration at a mechanical vibrationfrequency higher than a resonant frequency of the device, whereby theintensity of the proportion of the supplied radiation coming from thegratings is a measure of the vibration amplitude.